Deep sea secrets The giant squid has inspired legends about sea monsters, now a new genetic analysis reveals some of the enigmatic creature's secrets.

Although the giant squid Architeuthis is the second-largest invertebrate in our oceans, remarkably little is known about the animal that inspired the legend of the ship-devouring Kraken and the fiction of Jules Verne and Herman Melville.

Growing up to 18 metres in length, giant squid live in all the world's oceans except in polar regions.

Most of what is known about this elusive animal has been gleaned from remains that have been found in the stomachs of whales, washed ashore or caught in fishing nets. It has only recently been glimpsed alive in its natural habitat.

To delve into the secret life of the giant squid, an international team of researchers led by Thomas Gilbert and masters student Inger Winkelmann from the Centre for GeoGenetics at the University of Copenhagen has analysed the mitochondrial DNA of 43 specimens collected from around the world.

Cephalopod expert and Museum Victoria head of science Dr Mark Norman says the findings are very exciting.

"We're all blown away talking about a common species worldwide," says Norman who collected some of the tissue samples for the study.

"For quite a while we suspected there were three species of giant squid because their body shapes seemed a bit different between North Pacific, right round Antarctica and then in the Atlantic Ocean.

"The deep sea is the most common habitat on Earth and this genetic study shows it's all connected."

Ocean highways and bottlenecks

The researchers suspect part of the squid's low diversity may be due to the animal's lifecycle.

After they hatch, juvenile squid are carried along with plankton by shallow currents, then they move deeper as they age, says Norman.

"They are all swimming on highways between those huge ocean masses," he says.

"Either as juvenile stages or free swimming adults they're covering an enormous geographic range within the one species. So maybe it's like an albatross that covers the world in its lifetime."

Another factor that may have contributed to low diversity is revealed by a comparison of the squid's DNA markers to the fossil record of other molluscs, says another co-author Dr Jan Strugnell from La Trobe Institute for Molecular Science.

"There are no known fossils of giant squids," says Strugnell, who helped with the molecular analysis. "The way that fossils come into this study is to enable us to see when dates have occurred in the past."

Strugnell says the analysis shows the squid population went through a bottleneck followed by a rapid expansion between about 32,000 and 115,000 years ago, which coincides with the last ice age.

"If you compare the genetic diversity of this [squid] species with lots of other species, the genetic diversity is extraordinarily small," she says.

"We can't be sure, but [the ice age] may have changed the abundance of some predators that were competing with the squid.

"So the population has increased in size, but the genetic diversity perhaps hasn't recovered from that time."